1. Field of the Invention
The invention pertains generally to etching and, more particularly, to etching as practiced in the manufacture of information processing devices.
2. Art Background
The fabrication of devices such as information processing devices, e.g., integrated circuit devices, magnetic bubble devices, and integrated optics devices, involves etching patterns into regions, e.g., layers, of substrate material of different composition which are either incorporated into the device undergoing fabrication or removed during the fabrication process. Regions which are removed include, for example, layers of organic polymer resist. Such a resist layer is typically patterned by being exposed to actinic radiation through a mask, and then subjected to either a wet or dry etchant which selectively etches either the exposed or the unexposed portions. Regions which are incorporated into devices include, for example, layers of semiconductor material, metal, and dielectrics, e.g., silicon dioxide. Typically, a pattern is etched or milled into one of these layers by initially forming an etch or milling mask such as a patterned resist, on the layer, and then etching or milling the uncovered portions of the layer with a wet or dry etchant or a milling agent. Hereafter, for the sake of brevity, it is to be understood that the term "etching", as used in relation to materials other than resists, encompasses milling.
An important consideration in these etching procedures is control of etch depth. For example, overetching a layer of resist material (subjecting the layer to an etchant for a longer period of time than is necessary to etch through its thickness) is generally undesirable since this often leads to a loss of linewidth control during pattern replication, e.g., when using the resist as an etch mask. On the other hand, it is, at times, desirable to terminate etching at a desired depth within a homogeneous layer of material, or at the interface between two different layers of material, to be incorporated into a device.
Various techniques have been devised for monitoring etch depth. One of the most widely used (now conventional) etch depth monitoring techniques relies on the transparency to visible light (light having a wavelength ranging from about 400 nm to about 700 nm) of many substrate layers. (Such a layer is transparent to incident visible light if it transmits at least 5 percent of the incident visible light.) In accordance with this technique, visible light from, for example, a laser, e.g., a helium-neon laser (which emits light having a wavelength of 632.8 nm), is directed onto a bare, i.e., uncovered, area of the transparent (to the visible light) layer undergoing etching, and the intensity of the light reflected from the layer is detected and recorded as a function of time. Because the layer is transparent, the incident light is both reflected from the upper surface of the transparent layer and is transmitted, i.e., refracted, through the layer, as shown in FIG. 1. If the layer overlies a reflective surface, then the refracted light is also reflected upwardly through the layer, exiting the layer to interfere with the light reflected from the upper surface of the layer. As etching proceeds, the thickness of, and thus the optical path length through, the substrate layer being etched is reduced. Consequently, at specific thicknesses, destructive or constructive interference, which correspond to, respectively, a relative minimum and relative maximum in the recorded intensity-time curve, occurs. It is possible to relate the time intervals between these intensity extrema to changes in etch depth.
A primary reason for the wide use of the above-described technique is its compatibility with the conventional alignment procedure. That is, fiducial marks in substrates are employed to align resist exposure masks. The resist layer is formed over the substrate and thus commonly over the fiducial mark. The exposure mask is aligned with the fiducial mark by shining visible light (generally the very same wavelength of visible light used for etch monitoring) onto the resist layer (which, because of its transparency to the incident visible light, permits the fiducial mark to be detected). Since the ability to detect fiducial marks with visible light is considered essential, changes in the etch monitoring technique involving increases in the opacity of substrate regions overlying these marks are avoided.
While the conventional etch monitoring technique has many advantages, including compatibility with the conventional alignment technique, difficulties arise when the substrate contains more than one transparent region. For example, if a transparent substrate region undergoing etching is supported by a second transparent region which overlies a nonplanar surface, e.g., a stepped surface, then incident visible light will be refracted through both transparent substrate regions, reflected from the structures in the nonplanar surface, and transmitted upwardly through the two transparent regions. Interactions between light beams which have so been refracted and reflected often produce an unwanted interference signal which, in may cases, is so large that the interference signal from the substrate region undergoing etching is undetectable.
The conventional etch monitoring technique has other difficulties. For example, if the etch rate (of the region being etched) is constant, then the etch end point, i.e., the instant in time when the interface between the transparent substrate region undergoing etching and the underlying region is reached, corresponds to a frequency change of the intensity oscillations in the recorded intensity-time curve. But because this frequency change can, and does, occur at any point along an intensity oscillation (in the intensity-time curve), the etch end point is difficult to anticipate. Moreover, if the thickness of the substrate layer undergoing etching is less than, or is a relatively small multiple of, the etch depth change corresponding to the spacing between adjacent intensity extrema, then there will be less than one intensity oscillation, or a relatively small number of intensity oscillations, corresponding to the substrate region thickness. These two effects often make it difficult to accurately determine etch end point, which often results in undesirable underetching or overetching of the region being etched.
In an alternative etch monitoring technique, applicable to opaque or transparent (to visible light) substrate regions, visible light is directed onto an area of a substrate region (undergoing etching) shielded by a patterned etch mask. The incident visible light is reflected both from the etch mask surface and from the etch pit (or pits) being etched into the substrate region. At specific etch depths, there is either constructive or destructive interference, with consequences similar to those described for the previous procedure.
The alternative etch monitoring technique also has many advantages and is also compatible with the conventional alignment technique (the etch mask is typically transparent to incident visible light). However if the (transparent) etch mask itself undergoes etching during the etching process (as is often the case), then this results in an interference signal (produced by varying interactions between light beams reflected from the top and bottom of the etch mask) unrelated to the etching of the substrate region. This unrelated signal is often much larger than that associated with the etching of the substrate region, which again results in undesirable underetching or overetching.
The etch end point of a substrate region (the point in time when the region has been etched through its thickness)is readily determined if the thickness, and etch rate, of the substrate region are known (for example, if etch rate is constant, then the etch time required to achieve etch end point=thickness/etch rate). Thus, techniques have been devised for measuring the thickness of substrate regions. One such thickness measurement technique, which is widely used because it, too, is compatible with the conventional alignment technique, involves shining visible light onto the (transparent) substrate region whose thickness is to be measured. If the region is known to have a thickness less than .lambda./4n, where .lambda. is the wavelength of the incident light, and n is the index of refraction of the layer, then the thickness is readily determined from the intensity of the reflected light. (See, for example, O. S. Heavens, Optical Properties of Thin Films (Dover Publications, New York, 1965), Section 4.4.) Alternatively, the thickness is determined by shining visible light of different wavelengths onto the region (whose thickness is being measured), and measuring the intensity of the reflected light for each wavelength. At specific wavelengths, interference phenomena occur through the previously described mechanisms. The thickness of the substrate region is readily calculated from the observed intensity extrema as shown in, for example, F. Reizman and W. van Gelder, Solid State Electronics, Vol. 10, p. 625 (1967).
While the above thickness measurement technique has been found to be useful in many instances, difficulties arise when measuring the thickness of a relatively thin, transparent (to the incident visible light) region formed on, e.g., deposited onto, a relatively thick, transparent region. Typically, the thickness of the relatively thick region is first measured. Then, the relatively thin region is formed on the thick region and the combined thickness of the two transparent regions is measured. Finally, the thickness of the relatively thick region is subtracted from the combined thickness to determine the thickness of the relatively thin region. However, if the measured thickness of the relatively thick transparent region is in error even by a relatively small amount, a substantial error in the measured thickness of the relatively thin region often occurs. As a consequence, the relatively thin region often suffers undesirable underetching or overetching.
Thus, more accurate etch monitoring and thickness measurement techniques continue to be sought.